The long-range objective of the project is the successful use, in the therapy of cancer, of high-dose methotrexate and the methotrexate-cleavage enzyme, a carboxypeptidase, that is synthesized by a Flavobacterium sp. The efficiency of essentially homogeneous carboxy-peptidase preparations (2000-fold purified) is being evaluated in the murine leukemia L1210 system. The approach to the assessment of enzymic activity is relevant to the pharmacokinetics of high-dose methotrexate and considers the relative effects of methotrexate and carboxypeptidase on plasma folate levels. Studies to date have demonstrated that appropriately scheduled administration of the enzyme following high dose regimens of methotrexate decreases drug levels in plasma rapidly and effectively to concentrations below those that prevent DNA synthesis in sensitive cell renewal systems. It is hypothesizes that the appropriately scheduled administration of the enzyme alone or in a regimen with leucovorin after high-dose methotrexate therapy will selectively restore DNA synthesis in sensitive host tissues, and thus ameliorate cytotoxicity to increase the therapeutic effectiveness of methotrexate. In addition to a variety of experimental therapy regimens, other studies will explore possibilities of interrelationships between levels of plasma folate and methotrexate, specific intestinal enzymes and the inhibition and restoration of DNA synthesis in the small intestine.